1. Field of the Invention
The present invention relates to a miniature electric motor. This motor is used for automotive electric equipment such as air conditioners of automobiles, audio visual equipment such as CD (compact disc) players, OA (office automation) equipment such as copying machines, optical precision equipment such as compact cameras, electric equipment for home use, toys or the like.
2. Description of the Related Art
A miniature electric motor is extensively used in various fields in addition to the above-specified equipment and is demanded to be quiet by reducing noise of motor.
FIG. 5 is a cross-sectional view of a conventional miniature electric motor. In the miniature electric motor 1, a stator 5 is mounted on an inner circumferential surface of a casing 4, having a housing 2 and a cover plate 3. A rotor 6 is disposed in the interior of the casing 4 and a rotary shaft 9 of the rotor 6 is rotatably supported by bearing portions 7 and 8 mounted on the casing 4. Brushes 11 are slidingly engaged with a commutator 10 of the rotor 6. The brushes 11 are held by a brush holder 12 mounted on the cover member 3.
In some cases, the thus constructed motor 1 is used for driving, for example, a damper of an air conditioner of an automobile. In these cases, the brushes 11 are brought into sliding contact with the commutator 10 when the motor 1 is driving, so that the brushes 11 are vibrated and a noise (a mechanical noise) is generated from the motor 1.
If a frequency of the motor noise is out of an auditory sensation area, the noise is not heard by the human ears. However, in the case in which the motor noise is a sound of the frequency within the auditory sensation area and is in particular a high frequency sound (for example, a sound having a frequency of about 10 kHz), the noise becomes a high noisy auditory sound.
In the recent automotive vehicles, in order to seek more comfort driving, a higher quietness within the vehicle interior is demanded. In accordance with this, the reduction (ideally, removal) of the above-described high frequency noise generated from the miniature electric motor used in the automotive electric equipment is required.
Therefore, all kinds of technology have been proposed in order to reduce the motor noise. One of these approaches is that an elastic member such as a vibration proof rubber is provided in the interior of the miniature electric motor to reduce the vibration to thereby reduce the motor noise.
For example, in the motor 1 shown in FIG. 5, in some cases, the elastic member (not shown) is disposed on the brushes 11 per se or between the brush holder 12 and the housing 2. Thus, since the elastic member absorbs the vibration of the motor 1, the motor noise is reduced to some extent.
However, it is difficult to satisfactorily reduce the motor noise. Also, when the motor is assembled, the work for mounting the elastic member is troublesome and the number of the motor parts becomes increased. As a result, the manufacturing steps of the motor are complicated. Also, unless the elastic member is mounted in a predetermined position with high precision, there are some cases in which the non-uniformity takes place in the extent to reduce the motor noise.
In order to overcome the above-noted defects, an object of the present invention is to provide a miniature electric motor which is quiet with low noises without the elastic member for the vibration proof.
Exemplifying a miniature electric motor used in the experiments to be described later, it has been found out from the measured results of the noises that a high frequency sound having a frequency of about 10 kHz is generated.
Accordingly, the present inventors assume that a part vibrating at a certain frequency, which is in mutual relation with a frequency of the above-described high frequency noise, will be a source for generating the above-described high frequency noise.
Therefore, the vibrating condition of the cover plate 3 out of the parts of the motor 1 shown in FIG. 5 was measured from the outside of the motor 1. In the experiment, a number (for example, several hundreds or several thousands) of points are set up in a matrix form over the full outer surface of the cover member 3, and the vibration at each point is measured. The vibrating condition in which the cover plate 3 is vibrated is expressed in a three-dimensional manner through computer graphics on the basis of the measured data at the respective points.
Then, it is found that a central portion 13 of the cover plate 3 is vibrated at a maximum magnitude in a direction of a centerline axis CL2 of the motor 1 as indicated by a two-headed arrow E. This is similar to the phenomenon in which, when a drum is hit, its leather is vibrated to generate a sound.
In addition, the frequency of vibration of the cover plate 3 is about 10 kHz. This frequency is substantially the same as the frequency of the high frequency sound generated from the motor 1. This means that there is a mutual relationship between the frequency of vibration of the cover member 3 and the frequency of the noise, and means that the high frequency sound having the frequency, which is substantially the same as the frequency of this vibration, is generated from the cover plate 3 when the cover member 3 is vibrated.
In addition, when the central portion 13 of the cover plate 3 is vibrated, the rotor 6 vibrates in the direction of the centerline axis CL2 and there is fear that adverse affect would be imparted to the motor 1.
Thus, it is clear that a cause of the generation of the high frequency sound generated from the motor 1 is due to the vibration of the cover plate 3. Subsequently, an approach for reducing the vibration of the cover plate 3 is reviewed.
Since the central portion 13 of the cover plate 3 is vibrated at a large magnitude, when the central portion 13 is depressed from the outside of the motor 1 to avoid the vibration of the cover member 3, there is no high frequency sound. However, this is not used directly for the noise countermeasure by the motor per se.
On the other hand, it is effective to press a back side of the central portion 13 of the cover plate 3 from an inside of the motor. However, since the rotary shaft 9 and the bearing portion 8 are disposed inside the central portion 13, it is impossible to press the central portion 13 from the inside of the motor.
Therefore, the present inventors pay their attention to a brush holder fixed to the cover plate, and propose to strongly and forcibly press a portion in the vicinity of the central portion of the cover plate by this brush holder. Thus, it is possible to reduce the vibration of the cover plate thereby to reduce the noise of the motor.
In order to attain the above-noted and other objects, according to the present invention, there is provided a miniature electric motor comprising a housing formed in a bottomed hollow sleeve shape, a cover plate fitted in an opening portion of the housing, a stator fixed to an inner circumferential surface of the housing, a rotor disposed in an interior of the housing, a rotary shaft of the rotor being rotatably supported to one and the other bearing portions mounted on the housing and the cover plate respectively, and a brush holder holding brushes and mounted on the cover plate, the brushes being slidingly engaging with a commutator of the rotor, wherein a first projection is formed in the vicinity of a center of at least one of the brush holder and the cover plate, and the first projection is forcibly pressed at a predetermined pressure against the other of the brush holder and the cover plate.
It is preferable that the first projection is formed on the brush holder having elasticity and is pressed at the predetermined pressure against a back surface of the cover plate with a resilient force of the brush holder.
Preferably, the first projection formed on the brush holder forcibly presses a portion in the vicinity of the center of the back surface of the cover plate with the resilient force of the brush holder at the predetermined pressure, whereby the first projection is somewhat moved toward an interior of the motor, and under this condition the first projection is brought substantially into linear contact with the cover plate.
For example, while the cover plate is vibrated, a magnitude of vibration in a central portion of the cover plate is increased, whereas the first projection presses the portion in the vicinity of the central portion, at which the magnitude of vibration is increased, at the predetermined pressure, whereby the magnitude of vibration of the cover plate is decreased as a whole and it is possible to prevent a vibration of the rotor in a direction of a centerline axis.
It is preferable that the brush holder is fixed to the cover plate in the vicinity of an outer circumferential edge of the brush holder, and an opening portion for arranging the rotor is formed through in the brush holder in a center thereof, the first projection is integrally formed on an outer surface of the brush holder along an inner circumferential edge of the opening portion, and a plurality of slit portions are formed radially from the inner circumferential edge.
Preferably, the plurality of slit portions are arranged uniformly along an overall circumference of the inner circumferential edge to thereby divide the first projection into a plurality of segments, and each divided segment of the first projection is likely to be elastically deformed individually to exhibit a spring effect individually and is brought into pressing contact with the cover plate.
Also, it is preferable that at least one second projection is formed integrally on an end face opposite to an outer surface of the brush holder, and the second projection is forcibly brought into contact with the stator.
According to one aspect of the present invention, the brush holder has the end face on the opposite side to the outer surface on which the first projection is formed, and the end face is a surface which is continuous from an end edge of the outer circumferential surface of the brush holder and which is perpendicular to a centerline axis, and the end face is arranged so as to face one end faces of the pair of stators, respectively.
It is preferable that the second projection projects from the end face of the brush holder in a form of a partially spherical shape.
For example, in the case in which the stators are formed out of relatively soft magnetic material such as a plastic magnet or a rubber magnet, while the second projection is forcibly pressed against one end faces of the stators, the second projection deforms somewhat the stators and somewhat enters the stators, and under this condition the second projection is brought substantially into area contact with the stator.
According to another aspect of the present invention, in the case in which the stators are formed out of hard material, the second projection is brought substantially into point contact with the stator.
Furthermore, at least one third projection is integrally formed on an outer circumferential surface of the brush holder so that the third projection is forcibly pressed against the inner circumferential surface of the housing.
It is preferable that the third projections have elongated shapes extending in parallel with a centerline axis, each outer surface of the third projections forms a partial cylindrical shape, and the third projections are formed along an overall circumstance of the outer circumferential surface so as to project from the outer circumferential surface.
Preferably, the cover plate is fitted in the opening portion of the housing, and the outer circumferential surface of the brush holder is engaged with the inner circumferential surface of the housing, so that the third projections are brought substantially into linear contact with the housing.
Also, in a preferred embodiment, the cover plate is formed integrally out of the same metal as that of the housing or resin material, and the brush holder is formed integrally out of synthetic resin or glass fiber reinforced synthetic resin with insulating property.
It is preferable that a plurality of connecting terminals are mounted on the brush holder, the brushes are electrically connected to the connecting terminals, and end portions of the connecting terminals project outwardly from a surface of the cover plate to be connected to a wiring.
For example, when a predetermined part of the miniature electric motor is vibrated, a mutual relationship between a vibration frequency of the predetermined part and a frequency of a motor noise is identical or proportional relationship with each other.
According to another aspect, with respect to the mutual relationship between the vibration of the cover plate and the noise of the motor, the vibration frequency of the cover plate and the frequency of the generated high frequency sound are substantially identical with each other.
In a specific embodiment, the frequency of vibration, when the cover plate is largely vibrated, is about 10 kHz, whereas the motor noise generated at this time is the noisy high frequency sound with a frequency of about 10 kHz, so that the vibration of the vibration frequency of about 10 kHz is reduced down to thereby reduce the high frequency sound.
As described above, according to the present invention, it is possible to provide a miniature electric motor which is quiet with low noises even if an elastic member for vibration proof is not used.